12 – Run-length encoding

The module itertools has a very funky iterable called groupby. If you’re imaginative, you can use it for all sorts of things.

One possible use-case is to compute the run-length encoding of an iterable. All it takes is to go through the grouped iterable and then compute the length of each group:

from itertools import groupby

def run_length_encoding(iterable):
    for val, group in groupby(iterable):
        yield val, len(list(group))

Each group is a lazy iterable iself, so you can’t use len directly on it. That’s why you see len(list(group)) in the code above.

Here’s an example usage:

print(list(
    run_length_encoding("AAAB0AA")
))  # [('A', 3), ('B', 1), ('0', 1), ('A', 2)]